Evaluation of different parboiling treatments on properties of sorghum products

Abstract

The effect of parboiling on milling yield, cooking quality, sensory properties and structure of sorghum (Sorghum bicolor (L.) Moench) was examined. Cultivars differing widely in kernel characteristics were: (1) soaked overnight and boiled for 10 min (Soak-Boil or SB) and (2) brought to boiling point, soaked overnight and brought to boil again (Boil-Soak-Boil or BSB). Parboiled grain was dried prior to decortication by abrasive dehulling. Both parboiling procedures significantly increased the yield of decorticated grain while reducing kernel breakage. The BSB process gave significantly higher yields with less brokens that the SB process. Decortication yield of soft endosperm cultivars (P721) was increased by two to three times with parboiling, suggesting that soft, floury cultivars such as high-lysine varieties and soft, brown sorghums (bird resistant types) was converted by parboiling into grain with excellent processing properties. Parboiled, decorticated sorghum produced a light-colored, cooked product with firm, intact kernels, acceptable appearance and aroma. Sensory properties of the cooked, parboiled grain were rated as very acceptable. The parboiled grain required more moisture and longer cooking times to yield a cooked product with tenderness comparable to that of the non-parboiled cooked product. Protein, starch, fiber and ash levels were not significantly changed by parboiling. Crude fat content was reduced by parboiling. The amber colored parboiled kernels were harder, more dense and smaller than the unparboiled grain and had a translucent appearance. Annealing of the starch during the soaking step was indicated by x-ray analysis. The formation of an amylose-lipid complex during parboiling was observed. Brightfield and scanning electron microscopy techniques showed that structure of the starch granules were altered more extensively in the peripheral layers than in the center of the kernel. Protein bodies were swollen by the treatment and the protein matrix seemed to be "fused" or solidified after being transformed to a mobile state by the heating treatment. Parboiling in Mali with local sorghum and millet samples confirmed the improved decortication yields obtained in the laboratory. The process requires low-level technology that is available in villages in sorghum and millet producing areas. No specialized equipment is needed to parboil grain on a village scale. Parboiling may be a practical method by which shelf-stable, processed products from sorghum and millet can be prepared while enhancing decortication yields, especially for soft cultivars.